1. Field of the Invention
The present invention relates generally to printed circuit boards, and more particularly to printed circuit boards having surface mount electronic components.
2. Disclosure Information
One of the most common failure modes occurring in surface mount printed circuit boards (PCBs) is solder joint cracking, as illustrated in FIG. 1. This is an electrical failure which is caused when the solder joint 10 which electrically and mechanically connects a component termination 12 with its respective mounting pad 14 cracks due to thermal cycling stress induced because of the difference between the coefficients of thermal expansion of the component/termination and the mounting pad/substrate. Typically such cracks 16 initiate at the inner fillet surface 18 and propagate underneath the termination 12 and out to the outer fillet surface 20, thus electrically separating the termination 12 from its mounting pad 14. This problem is especially pronounced in such environments as automotive applications, where the PCB must be able to perform in conditions anywhere between -40.degree. C. and 125.degree. C.
Various approaches have been proposed for mitigating this problem, the most common being the use of higher melting point metal particles mixed into the solder to impart more thermal fatigue resistance to the solder. However, it is often difficult to consistently control the standoff height H using such an approach. Another approach is to use a leaded component rather than a leadless one, but this has the disadvantage of requiring more board space and often a higher component cost.
It would be desirable, therefore, to provide some means for improving solder joint resistance to thermal cycling fatigue and cracking without these disadvantages.